Officinal. Juniperus sabina from Central and South of Europe (the young branches yield an essential oil). The wood of J. communis is used in the production of an essential oil, and J. oxycedrus in the production of empyreumatic oil. The “berry-cone” of J. communis is officinal, and is also used for gin.—The wood of J. virginiana (N. Am.) is known as red cedar, and is used for lead-pencils. Sandarack resin is obtained from Callitris quadrivalvis (N.W. Africa).
The following are cultivated in gardens:—Thuja occidentalis (Arbor vitæ) (N. Am.), and orientalis (China, Japan); Juniperus sabina and virginiana; Thujopsis dolabrata (Japan); Cupressus lawsoniana (California), C. sempervirens (S. Eur., W. Asia), and other species, are grown especially in conservatories, and in Southern Europe particularly in cemeteries.—The Retinospora species which are so often planted, do not belong to an independent genus, but are obtained from cuttings, taken from seedling-plants with acicular leaves (see page [267]).
Class III. Gneteæ.
This class, independent of extinct forms, comprises the most highly developed of the Gymnosperms, partly from the circumstance that a perianth of 2–4 members encloses the terminally placed ovule, which is provided with one, or (in Gnetum) two, integuments, and partly owing to the fact that the wood has true vessels. There is only one order.
Fig. 275.—Welwitschia mirabilis (considerably reduced). The horizontal lines indicate the surface of the soil.
Order. Gnetaceæ. The three known genera differ very much in appearance. Welwitschia mirabilis (from the deserts of South Western Africa) is the oldest (?) genus now living. It resembles a giant radish, in that the hypocotyl is the only part of the main axis of the stem which becomes developed. It attains a circumference of upwards of four metres with a length of 1/2½-⅔ of a metre. It bears only two oblong, leathery leaves (Fig. [275]) which are torn into segments at the apex and lie on the surface of the soil; these are the two first foliage-leaves which succeed the cotyledons, and they are remarkable for their enormous length (upwards of two metres) as well as for their long duration, living as long as the plant itself. In their axils are situated the 4-rowed, spike-like male and scarlet-coloured female cones, upon dichotomous branches. The perianth consists in the ♂ of 2 alternating pairs of leaves, the inner ones of which are slightly united. The andrœcium likewise consists of 2 whorls: the external (transverse) with 2, the internal with 4 stamens; the lower halves of the 6 filaments uniting to form a cup. Each of the terminal anthers corresponds to a sorus of 3 sporangia, the sporangia being fused together, and opening at the top by one three-rayed cleft. In the centre of the ♂-flower there is a sterile ovule. In the ♀-flower a perianth of two connate leaves is present.—Ephedra (desert plants, especially in the Mediterranean and W. Asia) at first sight resembles an Equisetum; the stems are thin, long-jointed, and the leaves opposite, small, and united into a bidentate sheath; ♂-perianth of two connate leaves (median leaves); 2–8 stamens united into a column. Each anther is formed of 2 sporangia (is bilocular). ♀ mainly, as in Welwitschia. The seeds are surrounded by the perianth which finally becomes red and fleshy. There are 30 species.—Gnetum has opposite, lanceolate, pinnately-veined, leathery leaves. They are mostly climbers (Lianas) from Tropical Asia and America. The ♂-flowers have a tubular perianth, (formed from two median leaves) which surrounds a centrally-placed filament, bearing 2 anthers. In the ♀-flower there is a similar perianth, surrounding an ovule provided with 2 integuments. The perianth becomes fleshy and envelops the hard seed. 20 species.
From the circumstance of Welwitschia having ♂ flowers which, besides stamens, possess also a rudiment of an ovule, Celakovsky draws the inference that the earliest Gymnosperms had hermaphrodite flowers which from this structure became differentiated entirely into ♂-and ♀-flowers, with the exception of Welwitschia only, in which this differentiation was only carried out in the ♀-flower. This theory has so far been scarcely proved.
Fossil Gymnosperms.
The earliest continental plants which are known belong to the Cordaitaceæ, a group of plants which existed as early as the Silurian period; they were Gymnosperms, but it has not yet been determined whether they were Cycads or Conifers. The Cycads, even in the Coal period, were scarce; they attained their fullest development in Jurassic and Cretaceous periods, during which they were rich in species and genera, and extended as far as the Polar regions. In addition to these, Taxaceæ, Abietaceæ, and Taxodiaceæ appeared in the Carboniferous period. The Taxaceæ appear to have attained their culmination in the Jurassic and Cretaceous periods; Ginkgo appears in the Rhætic; Torreya, in the Cretaceous; Taxus and Podocarpus in the Tertiary periods. The Abietaceæ also appear in the Carboniferous; Pinus was first known with certainty in the English Weald and in the Cretaceous; almost all other contemporary genera are represented in this latter period. The Araucariaceæ first appear, with certainty, in the Jurassic. The Taxodiaceæ may be traced back as far as the Carboniferous (?); Sequoia is first found in the lowest Cretaceous, at that period it spread throughout the entire Arctic zone, and being represented by a large number of species, formed an essential part of the forest vegetation. Sequoia played a similar part in the Tertiary period. The Cupressaceæ are first known with certainty in the Jurassic, but they appeared more frequently and numerously in the Tertiary period, in which most of the present living genera were to be found. The Gnetaceæ, according to a theory advanced by Renault were represented in the Coal period by the genus Stephanospermum, which had four ovules enclosed by an envelope.